text:
Controllability vs. Stability in PHAK. Is it an error?
My understanding on the relationship between stability and controllability has been that greater stability means less controllability and vice versa. This post confirms that. However, I've came across this paragraph from PHAK 5-43 today:

Generally, an aircraft becomes less controllable, especially at slow
  flight speeds, as the CG is moved further aft (my emphasis). An aircraft that
  cleanly recovers from a prolonged spin with the CG at one position may
  fail completely to respond to normal recovery attempts when the CG is
  moved aft by one or two inches.

Aft CG means less stability, so if I'm not mistaken, the phrase is basically saying less stability leads to less controllability. How should I understand this statement? Should I understand that, although controllability and stability are generally opposed to each other, when it comes to a stall or spin recovery, the opposite is true? Or is it simply an error?

Your understanding of controllability and stability is correct, for the following definitions:

Controllability: the response of an aircraft in steady flight, on pilot control inputs. For instance deflecting the ailerons: a high resulting roll rate means a fast response.
Stability: the tendency of an aircraft air frame to return to trimmed position after a disturbance in air stream. Note that in this definition of stability, the airframe itself does all the work in getting back to neutral position, no pilot input required.

So the above is for steady, trimmed flight. The paragraph that you cite is about how to provide flight control inputs to recover from a spin. Now we need both control deflections and aerodynamic stability to recover. If the CoG is too far aft, there is not enough stability and the airframe may not recover even if the correct control inputs are given.
Use of the word Controllability is correct here, since it refers to controllability of recovery from a spin.

Then does this inverted relationship between stability and controllability applies only to a spin recovery, or more generally slow flights (including level flight) as the paragraph seems to suggest as I see it?

Yes, there is less of an aerodynamic stabilising moment at slow flight. Response to flying control inputs is also sluggish, reducing controllability.


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PIC flights during grace period
If my base month for recurrent 135 checks is June and I fly as PIC in July and subsequently don't complete required testing in July, can I be violated for any July flights?

Does this question help?

@pondlife this is a different question.  He is asking "what if I fly in my grace month and then never complete the ride."

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What is the future for auto-pilots?
What is going to change with auto-pilots on commercial aircraft in the next decade or so? Will airlines allow an auto-pilot to control an aircraft when it lands and takes off? Will we start using

Welcome to Aviation Stack Exchange!

Hi Kenzoid and welcome to Stack Exchange.  I like  where you're going with this question, but it's hard to answer without getting overly broad or debating opinions, and SE isn't the best place for that kind of rambling or speculation.  Maybe you could ask more specific questions like will airlines use autopilot for take-off?, as those questions would be great if they're not already answered.

Some good questions that may help answer what you're asking: How much of a flight could be fully done by autopilot? Why do we still use pilots to fly planes? or browse other auto-pilot questions

A decade is not a long time at all.
What the future holds for the next decade is the new aircraft-ATC communication system, CPDLC. All of the focus is on that. Enabling data links between pilots and ATC is a hard prerequisite for any significant autopilot development.
It's very possible and likely that nothing revolutionary will happen in autopilots.
It's possible and likely that something more minor is going to change.
Some believe autopilots will get more sensor and control fusion. Still nowhere near full control. Others believe more manual flying will become standard procedure, to reduce the damage when the electronics fail and reversion to direct law is required.
No one knows that for sure, so it would be pure speculation. What is known is that autopilot development will not be the next decade's primary focus. For any major new system to fly passengers in 10 years, it would have to be in advanced testing stages already.

99% of the time pilots spend on an airplane is basically automated.
Flying a strait line doesn't take much input other than making minor adjustments to account for wind direction ect...
Landing and takeoff however are the most important and difficult aspects of flying. Precision input is constantly applied to the controls with much less forgiveness when a mistake is made as there is no time to correct for it.
However computers are already very good at doing these procedures. In normal conditions it wouldn't even be a problem for a computer to land these planes. But that's the key, normal conditions.
What about an emergency? What about high wind gust? What about a hijacking? A birdstrike? When something goes wrong and programmers had no way of foreseeing the problem no computer could generate algorithms on the fly to deal with it. A trained pilot however just might be able to save the day.

The airline industry is very very strict about risk management and any changes to be considered must undergo extremely hard testing.
If a company came out tomorrow with an actual working piece of software that could fly a plane to its best possible abilities in any condition, and out perform humans in every single possible situation I'd reckon it would still take 10+ years of red tape just to start human trials on it.
And fyi we do not have that type of tech today. There have been great strides recently in terms of autopilots using techniques such as machine learning, but these practices basically started in the past decade and the knowledge we have is very young.
Self driving cars require human input anytime it gets in a situation it doesn't understand. A friend of mine owns a tesla and said his car requests input every 15 miles on average while driving on the outskirts of the city.
I tried to find better stats for an average but didn't see any from a quick google search, still though even his case of required input every 15 miles shows we have a way to go.
How much further do we have to go? I don't know. I don't think anyone does. It's like asking what kind of smart phone will we have in 20 years. Will we even have smart phones?
But one things for sure, before it would ever be allowed to happen, if ever, it would have to have a fail/pass ratio of basically 100%.

99% of the time on autopilot would leave just 90 seconds for takeoff and landing combined on the average 2.5-hour flight. That's if we don't count taxiing.

@Therac Ha ha you got me there, but it's a valid point. Pilots taxi planes on the ground, so any "replacement" autopilot would have to also be a self driving ground vehicle also to navigate through modern airports.

@Therac The exact limits depend on company policy, but it's not uncommon for the AP to be allowed from 500ft AGL after takeoff until 500ft AGL before landing. At the speeds airliners fly, that's probably less than one minute on each end, including ground roll. 99% is actually possible. Still, the tiny fraction where the humans are flying is the by far most challenging and where things tend to go wrong; I've read 97% of accidents happen within 2 miles of an airport.

@StephenS Who in their right mind would fly autopilot for climb and descent if they're allowed to fly manual? A lot of people, really, but most join the profession to fly... It's company policy and fuel economy that force more autopilot time. If fuel gets cheaper than reputation (lost when pilots with no manual skills end up making mistakes), even a mild reversion of that trend is possible.

@Therac That would make a good new question; the answer is far too complicated for comments. I was only addressing the 1% issue.

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What does a higher pitching moment mean? (ie -0.095 vs -0.0022)
I've found that as the angle of attack increases, the moment becomes more negative. I wanted to know what does that negative value say about the characteristics of the airfoil, and what is its importance.

A negative pitching moment is stabilising: it is a nose down pitching moment, which is what is required for static stability.
If the stabilising moment of the wing profile increases with AoA, the horizontal tail volume can be smaller - less resistance.

so to clarify, if the coefficient say is -0.50, it stabilizes more than a -0.01

Yes indeed, -0.01 is almost neutral.

Wait, no. You are talking about the pitch moment derivative per AoA ($Cm_{\alpha}$), not the moment itself. The derivative should be negative for stability, and its magnitude largely determines the 'amount' of stability; but the moment itself can be anything. Positive for negative AoA, for example.

@Zeus Yes the $C_M$/$\alpha$ of the whole plane must be negative, a result of the sum of wing, tailplane and fuselage moments. The $C_m$ of the wing profile is by definition constant in the aerodynamic centre. If the wing contributes more nose-down moment, the tailplane may contribute less. And indeed, total $C_M$ of wing, tailplane & fuselage at negative AoA will be positive.

It is not clear what OP is really asking then. For an airfoil, "as the angle of attack increases, the moment becomes more negative" is meaningless without a reference, yet at the standard reference (AC) it is indeed const. If he's asking about the negative $Cm_0$ (which doesn't change with AoA!) of a typical camber airfoil but in the context of the whole airplane, then you can't really say $Cm_0$ is 'stabilising': it's a bias, and if you put CG between AC of the wing and NP (which is quite normal), the wing will be destabilising regardless.


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Do airline pilots use FD when performing visual landing?
Permitting weather, most airline pilots get visual landing clearance in the US, unless they specifically have requested an instrument or RNAV approach due to their company rules or special circumstances. My question is: when they disengage Auto Pilot for visual landing, do they still use Flight Director until short final?

This answer will be quite anecdotal (I have not seen a survey on the question). It will answer the question for "most people I've worked with", and for "my" aircraft (B737NG). Also, the usual disclaimer: different airlines/aircraft manufacturers may have different SOPs/instructions/limitations. Consult your manuals.
In general: most pilots will leave FD on, IF it still provides useful and reliable information, without being an obstruction/nuisance (i.e. when configuring it to give correct guidance would be more trouble than it's worth, given that you already have visual references). This philosophy is also usually what you'll find in an airline SOP, but in more formal language.
I'll try to provide some examples (non-exhaustive) to illustrate this depending on how you were set up before being cleared for the visual approach:
ILS to straight in landing
Leave the FD on until landing (on my aircraft, B737NG, it will disappear at 50 ft RA), both localizer and glideslope will give usable and reliable information to the FD. (Caveat: in visual conditions, ATC might be less strict about avoiding signal interference. They might for instance let an aircraft on the ground taxi to cross your runway while you're on approach, which could cause localizer/glideslope interference).
ILS to sidestep
Most would turn the FD's off. Both localizer guidance and glideslope guidance will be to the "wrong" runway. The FD is likely to be more of a nuisance than a useful tool.
Non-ILS approaches with no centerline offset
The Localizer/LNAV path will give good lateral guidance to the FD. The vertical guidance will depend on the approach and your equipment. If the aircraft generates a good quality vertical path, which crosses the runway threshold at a reasonable height (50 ft), there is no real good reason to turn the FD's off. If all you have is vertical speed (V/S) guidance, it might be more helpful to turn FD's off.
Non-ILS approaches with centerline offset (for example most VOR approaches)
At some point during the approach, you'll want to abandon the approach course, and line up with the runway. Where this point occurs depends on specific approach, and how you intend to fly it.
Example: A VOR approach to runway 36 (runway heading 000) might use a VOR 1 nm east of the runway with inbound course 005. You might want to stay on the inbound course of the VOR until it crosses the extended runway centerline. In this case, most people would keep the FD on until reaching this point, and then turn it off, as it won't give you any useful guidance afterwards.

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What exactly are the "sealed" leading-edge slats in B737 short field performance aircraft?
"The 737 design enhancements allow operators to fly increased payload in and out of airports with runways less than 5,000 feet long. The design enhancements include a two-position tail skid that enables reduced approach speeds, sealed leading-edge slats that provide increased lift during takeoff, and increased flight spoiler deflection on the ground that improves takeoff and landing performance"
Source: https://boeing.mediaroom.com/2006-07-31-Boeing-Delivers-First-737-with-Enhanced-Short-Runway-Package-to-GOL
What are the sealed leading-edge slats in B737 short field performance, and how are they different from the normal leading slats in typical B737?

A "sealed" slat has no slot between it and the main wing.  According to b737.org.uk, sealing flaps allows for more lift at a lower AOA (Angle of Attack).
Reference from Flightglobal.com "Short Field 737 goes into flight test" states that sealing of up to 25 degrees of slats will be available for take-off, an improvement from 1 and 5 degrees.  Slots (not sealed) are to be used for landings.
This information  indicates "sealing" creates a large single undercambered surface, which would have a lift to drag (less drag) profile more suited for take-off and climb out than the slotted settings.


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What's the difference of Vs from Vs0 or Vs1?

Vs: Stall speed or minimum steady flight speed for which the aircraft
  is still controllable
Vs0: Stall speed or minimum flight speed in landing configuration.
Vs1: Stall speed or minimum steady flight speed for which the aircraft
  is still controllable in a specific configuration]1

How is Vs different from Vso or Vs1? Is it safe to assume Vs is a generic term involving both Vso and Vs1?

VS is the generic term.  However, since stall speed varies depending upon how the aircraft is configured (in particular the flaps), we break that down into VS0 for landing config and VS1 for cruise config.  In theory, we should list the stall speed for every possible configuration, but it's generally sufficient to list just those two because all others should be somewhere in between.

This is a good answer. If you get a moment, could you refine a little why "in a specific configuration" maps to "cruise config"? Is this canonically always the case, or does this vary from designer to designer?

@KennSebesta I’m not aware of any variance, and nobody has corrected me in 2.5 years. Few pilots will care about exact stall speeds for any configuration other than minimum drag, aka “clean”, since that’s where we spend most of our flight time, and landing, since that’s where we are flying slowest and thus at greatest risk of stall.

There's one more scenario where some of us care: studying for the FAA written. :D

VS0 and VS1 per Cessna 172 S POH
VS1 is most front C.G, flaps up with max weight (2550 lbs)

To help users that can't read/visualize the image, would you mind describing the content in words?


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Why is "LINING UP" used as the proper response to a "conditional" line-up instruction?

This is a list of the ICAO's standard phraseologies used for preparation for takeoff. I don't understand why "lining up" is (recommended to be) used as a response to a conditional line-up instruction. It can be very confusing, because it sounds like the pilot is lining up (against the ATC's intention) when he has to wait until the condition is met, like in the situation where the pilot has to wait until the arriving airplane touches down before she can taxi onto the runway to line up. I'm not a native speaker, so it might be just me. Does it sound reasonable to native speakers?

I can't answer your "why" question but as a native English speaker it does seem odd, for exactly the reason you mentioned. FWIW, when I learned to fly in South Africa I never heard anyone use it; I got plenty of conditional LUAW instructions but I always just read it back as "line up and wait". The US doesn't allow conditional LUAW so it isn't an issue here.

The language in the US used to be "Nxxxxx, position and hold, runway 24", for example. "runway" didn't need to be said as you were typically already holding at the runway hold short line.  And one would respond "Position and hold 24, Nxxxxx" then pull out on the runway and expect takeoff clearance next.  Apparently, we have now switched to international language (which I think of as "european") and the language has changed to "line up and wait", meaning get your plane on the runway center line and wait for takeoff clearance.

Both "line up" and "lining up" are okay for the non-conditional form, so perhaps they're both allowed for the conditional form as well to make things more consistent for non-native speakers. Even most native speakers would have a hard time explaining why "lining up" can't be conditional, just that it sounds wrong, so expecting non-natives to grasp that isn't reasonable.

@CrossRoads the "new" phrasing was developed after the Tenerife disaster and many close calls that have happened because instructions were mixed up or unclear. Hold means do not cross an intersection, position is just report your position. You don't want somebody pulling onto the active when ATC only wanted their current location or worse wanted them to hold without crossing. The same reason why "cleared" is never be used for a taxi clearance. Only takeoffs and landings use the word cleared in ground movements, "proceed to" or "taxi to" are used for ground clearance.

"Lining up" is not a proper response to a conditional lineup instruction;
"(condition) lining up (brief reiteration of condition)" is correct.
To use an actual example:
ATC: "RYR123 behind landing Airbus 320, line up runway 28L behind"
Pilot: "Behind landing Airbus 320, lining up runway 28L behind"
In my opinion, that leaves very little room for confusion.

So basically: you can't ignore the parentheticals in that list.


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How can pilots use receiver function of VOR during lost comms?
In the event of lost comms, are pilots encouraged to try the receiver of the VOR nearby if the VOR provides the function? If they are, what's the proper use of the VOR receiver? Just tune in the VOR frequency in the VHF radio in the cockpit and listen to what ATC says and follow it? Does ATC expect pilots experiencing lost comms would use the VOR receiver as an alternate means of communications?

VOR carrier frequency can be modulated by voice in addition of being modulated by the VOR angle signal and Morse ID. Only ATC can use such possibility, and not all VOR are capable.

@min I'm aware that not all VORs have the function, and even when they do have the function, only ATC can transmit through them. My question is: Does ATC expect pilots to monitor the frequency when they're experiencing a loss of communications?

"Does ATC expect pilots to monitor the frequency when they're experiencing a loss of communications?" This is answered, maybe too superficially, with reference to documentation for both ATC (JO 7110.65 10-4-4) and pilot (AIM 6-4-3).

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How are IRUs and AHRSs different in terms of their functions?
By my understanding, both systems use gyros and accelerometers to provide aircraft's attitude and heading information. Given that the information they produce are more or less the same, why do the modern aircraft have them both? What are major functional differences of the two systems?

What an AHRS can do is a subset of what an INS can do.  Both will give you attitude & heading; the INS also gives you position and velocity, which the AHRS doesn't.  An INS is considerably more expensive as a result, but allows you to continue to navigate through losses of GPS signal.  An AHRS is a solid-state replacement for mechanical gyros, but it doesn't keep track of your location.


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why are tips of airplane wings "bent' up?
Why are the tips of wings on new aircraft folded up? Is it safety or efficiency that is the concern? I have done multiple searches and all I ever seem to get are wingtip shoes!

Yes, search on "winglets" not "wingtips" and you will find a bunch of info.

Welcome to Stack Exchange! The page that Greg linked to may answer your question. If it doesn't, please let us know what questions you still have after reading it. Thank you!


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Are airliners' cruising altitudes we know indicated or true altitudes?
Are airliners' crusing altitudes, which commonly range from 25000 to 45000 feet depending on various conditions, indicated altitudes or true altitudes? If they are indicated altitudes, what are the range of cruising true altitudes?

What do you mean by "indicated altitude"? We have "indicated airspeed" and "true airspeed". I believe the term you're looking for is "pressure altitude".

@kevin Although you're right in saying that the indicated altitude is the pressure altitude at crusing levels, there IS indicated altitude. It's a known concept.

Note that there are 3 kinds of altitude: geometric altitude (measured by GPS), indicated altitude (measured by barometric altimeter set to local reference datum) and pressure altitude (measured by barometric altimeter set to 1013 hPa (29.92 inHg)). The first two are different and the third is different unless local reference datum happens to be standard pressure.

The delta between "indicated altitude" and "true altitude" depends on how much the actual pressure differs from the set 29.92.  If the actual pressure is 29.92, then indicated = true altitude.
For each 0.01 of altimeter setting, you change the altimeter reading by about 10 feet; for each 0.10 of altimeter setting, 100 feet, and for each 1.00, 1000 feet. So if your actual pressure is 30.92, then with 29.92 set in your altimeter, you'll be 1,000' higher than indicated, so at FL410 (read on your altimeter with 29.92 set), your "true altitude" would be about 42,000' MSL.
Anything approaching 31.00 as an altimeter setting is pretty rare, so I'd say that 42,000' is about the top end of the range for most commercial operations, although if you find a jet that can cruise above FL410 (at least some models of the 747 can, IIRC), then add 1000' to their max ceiling for your answer.
As far as the low end, anything below about 29.00 is a pretty rare altimeter setting, so subtract 1000' from whatever you want to use as your range for "indicated" altitude, and that will be the bottom of your corresponding range of "true" altitudes.
That all said, I've used altitudes below FL 180 for a cruise altitude in airline operations plenty of times, so there my indicated would essentially match my true altitude. Most often, this is on short flights where you can get more direct routing by staying low & out of more congested airspace up higher (U.S. east coast); sometimes the headwinds/tailwinds or the rides (turbulence) is enough better down low to make it worthwhile to fly below the flght levels.
So, if you want to consider what is the range of commercial operations, I'd say that FL410 is mostly a very firm upper limit, with very few airline flights above that, while the lower limit would be much less clearly demarcated. And converting to true altitude, add/subtract about 1,000' from your indicated (flight level) altitude to cover almost all of the likely variation in atmospheric pressure.

Thank you so much. May I ask you a follow-up question? When a jet is certified for cruising altitudes up to 45100 feet (B747), is it pressure altitude?

Always a Flight Level, as far as I've seen. For example, the 737 is good to FL410, which isn't changed no matter how high or low the.local pressure may be.

The difference between “indicated altitude” and “true altitude” actually depends on the actual temperature, humidity and bunch of other atmospheric properties that are never corrected for. The difference you describe is between “indicated altitude” (corrected for local pressure (but not density)) and “pressure altitude” (measured with standard setting of 1013 hPa (29.92 inHg)). “True altitude” is measured with GPS and only used for terrain proximity warning.


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What's the purpose of depiction of visual flight track on some Jeppesen approach charts?

29 is the visual flight track in this chart. Why is it depicted from the MAP to the threshold of the runway? Does it signify some alternative procedures pilots are expected to perform if they decide not to perform a missed approach at the MAP?

What is the image source?

@Ron Beyer I googled it but you can find it from the chart legend in the Jeppesen Airway Manual

Looks like it's a non-precision approach (since it shows MDA) so I think you can stay at that altitude until you see the runway. It's not like a DA where you go missed if you can't see it right away. I could be wrong,though

@TomMcW Thank you for your answer but that can't be true. If you don't have the runway in sight after you have reached the MDA, you should initiate the missed approach at the missed approach point, which is depicted as "M" on the chart. You simply cannot stay at the MDA once you go past the MAP


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Why is stabilator used in some small airplanes?
What makes it so attractive to use a stabilator in some small airplanes like Piper Cherokee or Seminole? Its primary purpose is to allow the pilot to control a pitch movement with a less force (which I presume better serve big airplanes), and if so, wouldn't a horizontal stabilizer coupled with an elevator be a better choice for the manufacturers, especially given that a stabilator installed in small airplanes typically requires an antiservo tab to prevent overcontrol by the pilot?

Simplicity. Yes, a servo tab is helpful, but most control surfaces should include a trim tab, so this can double as a servo tab. Total part count should still be lower than that for a stabilizer-elevator combination. But there are other advantages: Easier adjustment of stick forces and little loss of stability with free-floating controls.
By hinging the stabilator (or full-flying tail) near the quarter point, its control forces can be kept small and tailored with the servo tab. Also, this helps to keep the hinge moment coefficient derivative over angle of attack $c_{r_\alpha}$ low. The ratio between it and the derivative over deflection angle $\eta$ determines the angle to which the control surface will float, and a small $c_{r_\alpha}$ means that the stability with a floating control surface will be close to the stability with a fixed surface.
Also, balancing masses are easier to place when the full surface moves.


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Can you climb at Vxse when you cannot at Vyse in a multi-engine aircraft?
Let's assume that you took off in a seminole and had one engine suddenly failed, say, at 400 feet AGL. You right away pitch for 88 knots (Vyse) to keep climbing, yet you start to descend because the density altitude is too high. In this situation, would pitching up for 82 knots (Vxse) give me a chance to climb back? Or would it only give me a greater descent rate?

Vyse is your best rate.  If you cannot climb at your best rate you cannot climb.  If Vyse gives you -100fpm, Vxse  will give you less than -100fpm (a faster descent).

I'd recommend reviewing Chart 5-23 from the PA-44 POH - Climb performance one engine operating - gear up for specific numbers based on aircraft weight and atmospherics.

If you're not seeing a positive rate of climb at your best rate of climb, single engine airspeed, then you're not going to see it at the best angle of climb airspeed single engine either.  By definition, Vyse is the best rate of climb that you're going to see on a single engine; any other airspeed, including Vxse, is going to give you a lower rate of climb, or greater rate of descent if a positive rate is not possible.  And attempting to acquire and hold Vxse you're getting close to the reverse side of the power curve, combined with ever decreasing rudder authority, it would be easy to have the aircraft drop below Vmc and depart from controlled flight.

Thank you for your answer. I posed the same question to my multi instructor lately and he told me in theory no airspeed would give you a climb if Vyse can't, but he added I should do whatever is necessary in emergencies and pitch for Vxse if it gives me a climb, which got me confused. Thanks.

Vx increases as the thrust margin reduces until it equals Vy at absolute ceiling (and then for gliding, Vx, as best glide angle speed, is higher then Vy). This works the same with both engines and single engine, just the values are different.


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(B737s) Why is the automatic activation range of the AUTO mode in flight limited to "between idle and 5%"?

This is the explanation on the Engine Start Switch in the 737 series FCOM. What might be the reason why the automatic activation range of the AUTO (or OFF) mode in flight limited to "between idle and 5%," rather than simply to "below idle" as in the CONT mode? Is there any good systemic reason for the limitation?

For those of us unfamiliar with turbines, can you give an approximate expected Idle N2 percentage?

The reason to go automatically to ignition below idle N2 is to try to avoid a flameout. If the engine has slowed down to be at or below 5% N2, it is way too slow for combustion to happen - you don’t add fuel to a normal start until about 25% N2, by comparison. So “below idle in flight” almost is a valid characterization of when AUTO will turn on the igniters, it is just that the bottom of that range stops not at zero but at a slightly higher value. Ignition isn’t going to accomplish anything between 0% and 5% N2 anyway. (Probably can’t accomplish much at 6% or 7% or etc either, but those are probably seen only briefly on the way to, or from, near zero, so the engineers picked 5% as their cutoff.)
Note also that some switches are

GRD-OFF-CONT-FLT

While others are

GRD-AUTO-CONT-FLT

So, for the switch with an “AUTO” position, there is no other “OFF” position, so there needs to be a cue to the system to turn the igniters off, rather than to let them run continuously on a shut-down motor. And rather than saying simply that OFF happens at 0.0% N2, they’ve specified a range, so that the igniters can be turned off for a shut-down but still slowly turning (windmilling N1) motor.

Thanks, but for an engine shut-down, you need to pull the engine starter switch to "CUTOFF" anyway, so it makes no difference whether the AUTO (OFF) mode provides igntions below 5% or not.

@lemonincider That’s true, and I can’t think of a case you’d intend to leave a shut-down motor with the start lever in idle for any great length of time. Maybe it could happen inadvertently with distraction & task saturation & trying to sort out an arrival, approach, and landing. In any event, if you did, the AUTO mode would then work like OFF even with a small residual N2.

Then why does AUTO turn the igniters on if N2 is below 5%, rather than off?

It doesn't. "Idle" means the N2 expected when the engine is running at stable idle. So if that N2 is XX%, then Auto turns the igniters on from XX% down to 5%, and off when N2 is below 5%. Note that "0%" isn't mentioned in the quoted text.


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Does extension of flaps 1, 2, 5 or 10 increase descent rates?
Let's say you need greater descent rates than you can get with the full use of the speed brake in an airliner. Does it help to use the above flap positions in getting greater descent rates?

General operational rule, as my training colleagues used to say: „Flaps are not speed brakes“. Still, more drag means steeper descend gradient capability. What scenario(s) are you after?

The scenario I had in mind was you need a greater descent rate to meet an altitude restriction for STAR when you were belatedly instructed by ATC to continue descent after a level-off.

@lemonincider While one should make every reasonable effort to comply with controller instructions, doing show should not entice you to exceed limitations or, for that matter, your own comfort level. That said, if you're comfortable with it and below max gear extension speed, you could consider doing that. You can always tell ATC you're unable, and you can always suggest an alternative, like maybe a 360. Also, you can be pro-active. If you think the controller is behind the curve, you can request lower. Don't be afraid to negotiate.

In your scenario, deploying flaps to steepen descent might be technically possible, but is operationally unfeasible for most types, as on the STAR most often the speed is kept high enough to fly clean for most aircraft types. This means that for most types, at best, slats can be used (they come out before the flaps and withstand higher speeds), and they won’t make much difference. Notifying/negotiating with ATC to vary speed and/or course are the methods of choice in your scenario, if speed brakes don’t cut it.

As @Radu094 said, a spoilers out, high speed dive will generally get you the highest rate of descent. For that max rate of descent you would put your airspeed right up against the limit.
Using the 747-100/200 as an example, that max airspeed is 350 knots as I remember, and you don't have the option of also using flaps because operation of the speed brakes with any degree of flaps is prohibited.
Even if the speed brakes prohibition wasn't there, the max speed on a -200 for flaps 1, 5,  10 is 275, 250, 238 respectively, considerably lower than the 350 knots for the max rate of descent with a clean wing.
You could extend the gear. Speed brake usage with the gear extended is allowed, and the max gear-extended speed is 320 kn. Though 30 kn slower than than the max speed without the gear, all that drag allows you to put the nose down further. I can't remember how the rate of descent in this configuration compares with gear-up 350 kn, but you'll have a much steeper angle (and a lot more noise), and you really come out of the sky.
There's a trade-off though. While the max gear-extended speed is 320 kn, the max gear-operation speed is 270, so you have to first slow down before using the 320 kn speed.

That and all you really risk in extending the gear past 320kts is damage to the fairing doors.  I guess that’s a risk which may be appropriate, depending on how dire the emergency is.

I've also heard using flaps and speed brakes together is not recommended but couldn't come up with a reasonable explanation. If you don't mind, could you explain why the prohibition is there? Is it a manufacturer's limitation or just a common op specs rule?

@lemonincider It's a design limitation determined by the manufacturer. You could design for higher flap extension speeds, but that would mean increasing the weight of the structure to be able to handle the higher loads, and the size of the flap extension motors to move the flaps against the higher airspeeds. The design goal for flaps should include getting the weight as low as possible without incurring normal operational difficulties.

@Terry Thank you very much for your reply but I guess my question was not clearly understood. The question I wanted to ask was not why flap speed limits are there but why use of flaps and speed brakes at the same time is prohibited.

@lemonincider I'd suggest asking that as a separate question in those words. I'm not an aerodynamicist, but I suspect the answer lays in the fact that their use together produces undesirable flight characteristics. I once saw a captain go to approach flaps while having forgotten he had the speed brakes out. He was capturing a glideslope from above. Bringing the power up did not stop our sink, and adding much more power and bringing the nose up more got us the stall warning clacker at a much higher speed than one would expect. Stowing the speed brake stopped the sink and the clacker.

It depends what you intend to do with the speed. If all you are looking for is absolute descent rate:  a high-speed, spoilers out and high speed dive will get you the best rate.
What you are usually interested in a normal approach however is not descent rate (vertical speed) as much as descent angle. In this respect, all/any flaps will decrease the L/D ratio, which is going to increase you descent angle. Each stage of flaps will have increasingly higher descent angles, but mind you at a lower speed.

Flaps improve lift and pitch angle, but they are not used to control descent angle on approach.

uhm... Granted that’s not their main purpose, but actually they can be used exactelly for that.(I do it almost daily). Be carefull when saying ‘flaps improve lift’, that can trick you into the wrong conclusions. All flaps will add more drag for the lift they provide. Since glide ratio is basically L/D , you glide angle will always be steeper with flaps. So when high on approach, slow down and lower some flaps... you will be surprised !


text:
Is there any equation on propeller thrust that takes into account the number of blades
I had a look at both
What is the equation for calculating static thrust?
and
Is there any equation to bind velocity, thrust and power?
However, it does not mention how much would the thrust be affected by the amount of blades. e.g if it was a 20 blade propeller with a given RPM, how much would it affect the amount of thrust
Could you please kindly help me with this? Thank you

Hi and welcome to the site! The term you are missing, and why your searches are not yielding results, is solidity, and it indirectly represents the number of blades.

Adding blades is actually bad for thrust. The only time you really need "more blades" is to reduce loading. Ideally you'd only have one blade on a propeller, but that has its own issues. The reason solidity is used in a formula is because it makes a reasonable approximation. The problem with blades is the interaction with them, so adding more creates aerodynamic issues (since blades operate in the wake of the proceeding one). A single formula doesn't scale that way.

The most simple propeller thrust equations replace the propeller with an abstract disk in which the flow is accelerated. How this disk is composed in detail does not matter. However, more elaborate codes which also model the helical wake of each blade will use blade count specific algorithms.
So the answer is yes, there are algorithms which account for the number of blades, but they are more complex than a single equation.
This answer shows how the number of blades of the propeller for the Supermarine Spitfire grew with engine performance, and this answer sheds light on the number of blades of helicopter rotors.

Hi Peter, thanks for the answer. Could you please point me into some reading about the more complex algorithms. And where would be the best place to find a list of propeller/turbine characteristic table? The propeller that I had in mind has more blades like a turbine engine without the compression&Combustion part (pretty much an 100%bypass turbofan if that makes sense).

@Scriptbaby: Something like this, maybe? For algorithms, I recommend Larabee, Liebeck or directly XROTOR by Mark Drela. Caution: It assumes lightly loaded propellers, and those with many small blades need more intricate codes.

kaempf compared to the fantrainer, i want more blades. How about if we see it from a turbine engine design point of view? E.g Rolls, PW, GE. How would they, "roughly" calculate how many blades and what pitch are required for their fan? Sorry if this does not make sense

@Scriptbaby: It does make complete sense. Those fans are specific for very powerful engines which need to avoid large propeller (= fan) diameters, so they compensate with a high solidity. Make no mistake, they buy the capacity for a high thrust / mass flow ratio with a loss in efficiency.


text:
Effects of Gravity on Glide Ratio
I know that the weight of an aircraft has no effect on the glide ratio. However, gravity affects not just weight but also downward acceleration, which presumably should have some sort of effect on the behaviour of a craft. Thus the question: how does a craft's glide ratio scale with a planet's gravity (assuming other environmental parameters like atmospheric density, temperature &c. are the same for the sake of the case)?

In a glide, the sink speed constant. Hence, the downward acceleration is zero...

Let's hope that space.stackexchange.com finds something suitable to link to here!

"gravity affects not just weight but also downward acceleration"  What do you mean by this?  Weight IS downward acceleration as far as I understand it.

@MikeB A litre-sized cube of tungsten has about 20 times the weight of a same-sized cube of ice, but has the same downward acceleration (about 9.8m/s²) in free fall (which is then reduced by air resistance, if there is an atmosphere in the environment). But a litre-sized ice cube on Mars (where gravity is lower) would have a not only a reduced weight, but also only have a modest downward acceleration of about 3.7 m/s².

@vicky_molokh  I still don't get what you are talking about. The change of weight, and the change of acceleration are identical, because they are both effectively measures of the same thing, ie the change in gravity.

If there is no gravity, there is nothing to worry.

@MikeB I apologise, I don't understand how are they identical. Could you elaborate using the example provided (two cubes of different mass but identical size [ice and tungsten], and two planets with different gravities [Terra and Mars], with the corresponding results of weight and acceleration for the four possible combinations)?

From this answer: glide angle tan $\bar\gamma$ = $C_D/C_L$. The weight, density, speed and wing area have all been divided out.

So on a planet where the two lift&drag equations are valid, the glide ratio is purely proportional to the lift/drag ratio - gravitation, no matter what magnitude, has been divided out.

The local gravity affects an aircraft through its weight -- nothing else.
If the weight (or gravity) would change, the speed for best glide (best L/D) would be different -- but the glide slope would remain the same.
That said, the atmosphere of this other planet would also likely have a different density (and also composition and even viscosity) that would have a greater effect.
The density would also change the speed for best L/D, but not the value of best L/D.
A different gas would first change the Reynolds number, which might change the drag behavior slightly -- and slightly change the L/D.
A sufficiently different gas would have a different speed of sound -- which could mean that compressibility effects would become important at a much lower speed.  However, temperature also affects this and is probably easier to imagine a very cold atmosphere having a more straightforward effect.

I don't know, but I suspect, that a wide range of atmospheric densities and gravitational accelerations are possible, and that there is considerable latitude for them to vary independently (consider just Venus, Earth and Mars.) If so, I further suspect that there could be some significant differences in Reynolds number at flying speed, leading to something more than just a slight change in L/D (but +1 for being the only person so far to mention the Reynolds number.)

On Earth, its relatively small variations in gravity have no effect on the glide ratio.
But you're correct in that a major change in gravity will, of course, affect aircraft flyability. But it will not do so by changing the L/D ratio directly. Instead, a major increase in gravity (e.g. 10x) will require much more lift to be produced.
So, flight will require much more power - 10x gravity means 10x lift, means 10x drag, means 10x thrust to overcome it, and thus 10x the fuel consumption per mile.
If air density is unchanged, this will require wings that normally have a higher lift coefficient, which means much more camber, a thicker wing, and/or a higher angle of attack. Any of these greatly degrade the L/D ratio. Flight range will be shorter (about 10x), or some speed might be traded for range, e.g. planes will be 2x slower and fly 1/5 as far.
If air density is increased by the same factor as gravity, drag will increase greatly. But powered flight will remain possible at the same lift coefficient, and with engines of comparable construction. It will be much slower, however, by a factor of about 3x. Range will also decrease, but not as much, if lower speeds are acceptable.

Flight on Mars is more difficult than on Earth because of the thinner atmosphere, but was possible while there was an unbroken helicopter on the planet.  I would think an increase gravity would likely increase atmospheric density so as to make flight easier, or allow lighter-than-air craft to be smaller.

@supercat Air density isn't directly proportional to gravity; Venus has less gravity than Earth, but a much denser atmosphere. With a dense atmosphere, flight will be easier. But more gravity is always worse for flight; it's more lift to produce, and thus more drag.

Doubling gravity while keeping the same thickness of atmopsphere would double atmospheric pressure and thus atmospheric density.  I think that if atmospheric thickness is limited by the boil-off of gas into space, having stronger gravity will allow for a thicker atmosphere.  If an atmosphere is thin because there was never enough gas to surround a planet or moon, stronger gravity would lead to a thinner atmosphere.  If there's too little gravity to maintain an atmosphere, gliding will be impossible no matter how little lift would be required.

I will analyse only the second part of your question since the first part has already been discussed in the other very good answers.

However, gravity affects not just weight but also downward acceleration, which presumably should have some sort of effect on the behaviour of a craft.

Correct:
On a glider you normally want to fly either maximising:

the range or
the time airborne.

To maximise:

the range, you have to fly at the best (highest) $L/D$;
to maximise the time, you have to fly at the best (lowest) sink rate.

Without entering in mathematical details, if we express lift and drag with the ubiquitous:
\begin{equation}
\left\{ \begin{aligned}
  D=½\rho V^2 S C_d\\
  L=½\rho V^2 S C_l\\
  C_d=C_{d_{0}}+kC^2_l
\end{aligned} \right.
\end{equation}
we get that:

the speed for the best $L/D$ (max range) is $\sqrt{\frac{2W}{\rho S}\sqrt{\frac{k}{C_{d_{0}}}}}$
the speed for the best sink rate (max time) is $\sqrt{\frac{2W}{\rho S}\sqrt{\frac{k}{3C_{d_{0}}}}}$

Note how these two equations are very similar except for a "3" in the speed for the best sink rate (which is therefore 76% the speed for the best $L/D$): a sailplane pilot will typically fly at the speed for best $L/D$ (max range) when looking for a thermal and then will decelerate at the speed of best sink rate (max time) when inside a thermal.
To come at last to your question: the weight $W$ does have indeed a direct impact on those two speeds. If we bring our sailplane on a planet with half the earth gravity then its glide ratio would be the same but the speed for the best glide ratio would be 30% smaller. Obviously half gravity would also have an indirect impact on the aircraft itself, as already explained for example by Therac's answer.


text:
What are the benefits of raked wingtips over tapered wingtips?
This post shows the differences between winglets and raked wingtips. I'm curious why raked wingtips have been developed when there are tapered wingtips. What are the benefits of raked wingtips over tapered wingtips?

Did you have a look also here? https://aviation.stackexchange.com/q/14767/1467 https://aviation.stackexchange.com/q/8544/1467

@Federico Thanks for the link. So should I conclude they are just different designs for different wings and have no meaningful difference?

That would be my take on it as well, yes, but if you feel you would like more clarifications on that, you can link in your question also the ones I linked to you and point out what you would like to have clarified.

There are quite a number of questions on this site about wingtips / winglets already, with very detailed answers. I feel they have already addressed your questions. If not, please feel free to clarify.


text:
Why is polar curve of a glider dependent on flight load?
This is my first question here.
I'm just a glider in education phase and not yet a pilot, but because I'm also physicist, I'm interested in some details more than other people.
My question is simple:
I know what a polar curve is. Why is this curve dependent on the wing load?
For my knowledge in aerodynamics the polar curve is a property of the airplane/wings, but not dependent on flight state. Otherwise I know from my flight instructor, that gliders are often filled with water to increase weight, which gives a better gliding ratio.
What I mean you see on this curve: why is the curve changing with wing load?
My only answer would be, that wings are flexed differently under different loads and that the different curves arise from (slightly) different geometries. Is there something else I have overseen?

I believe the answer to your question is that the speed polar changes with wing load, because you achieve the same „flow geometry“ (angle of attack) at a different speed. The dimensionless lift/drag polar in coefficient form (which is the one purely dependent on aircraft geometry, unless aeroelastic effects are considered) will not change appreciably with wing load. Not entirely sure though whether that answers your question, so posting as comment only.

Related

see my mistake. It is rather embarrassing...

Hi @CptReynolds I like the Youtube channel!

@Jihyun not sure what you mean? I don’t have one...

Oh what never mind coincidence then sorry :P

Oh god- I made a terrible mistake: While $C_{lift}$ and $C_{drag}$ are really independent on weight, this is (of course..) not the case for the velocity polar curve. The two curves scale perfectly with weight and are just subject to a simple transformation. No magic behind and completely transparent.
Sorry: Question Solved ;-)

You say: "Otherwise I know from my flight instructor, that gliders are often filled with water to increase weight, which gives a better gliding ratio." But then the heavier glider has a different polar curve, right? And it has: Read here why.


text:
How do I calculate new minimal speed for a glider when I put water into the tank?
Lets say a glider weights 400kg and has a minimal speed of 70km/h.
When it has additionally 100l water, so in total 500kg, what is the new minimal speed?
I would argue, that in order to reach the same point in the polar diagram, we have to scale according to
$v_2 = v_1 \cdot \sqrt{m_2/m_1}$
because drag and lift forces scale quadratic with speed.
The result would be 78km/h.
However, in the theory we learn
$v_2 = v_1 \cdot m_2/m_1 = 87km/h$
Q: Isn't that wrong?

If the water tank(s) are original equipment on the glider, you will always find the answer to this in the operating handbook.

That's clear, but doesn't answer my question...

Fair enough. So let's go back to the  beginning. What do you mean by "minimal speed"? I am a glider pilot and we don't use that term. Do you mean stall speed? Or ??

Yes, that's wrong. Your calculation is correct. Where did you find the equation you refer to as being "in the theory"?

Lift is related to airspeed, all other things equal, by v squared. Hence, your calculation is OK, and that 'in the theory' is clearly wrong...

The "theory" appears to be regarding momentum, not used to calculate lift.  Lift is proportional to the square of speed.  Speed squared is also applicable to the kinetic energy state of your aircraft, which can put you on    the far side of the runway if you land too fast.

It might be a safety thing. You can probably do that calculation in your head, but I bet most people don't take calculators when they go flying, and square root would be harder to calculate. The aim of the minimum speed is to prevent you from crashing, so if the theory gives you a higher one, and an easier equation to use on the field, you are safer.

$v_2 = v_1 \cdot \sqrt{m_2/m_1}$ is the correct formula.
$v_2 = v_1 \cdot m_2/m_1 = 87km/h$ is simply wrong, possible that it is only a typing mistake, but it is wrong just the same.


text:
What is a two- or three-mile base exactly?
I frequently hear the tower instruction

Report a two mile base for runway 5

What is the intention of that instruction? I realize that under these circumstances that we're not looking for pinpoint precision, but still it would be nice to know.
For the record, I'm not asking what a base is (or the fact that "left" is implicit), but what two miles are we talking about? Two miles straight line distance from the threshold? A base to a two mile final? On the base leg, two miles from the centerline? (I'm pretty sure the last one isn't it, but hey, I've been wrong before)

There are several ways to enter the pattern at a controlled field. The most common are right/left pattern and straight in. If you are approaching perpendicular to the runway and downwind of the approach end the controller will often clear you for a base-leg entry to the pattern (skipping the downwind) . What they want to know is when you are two miles from the extended centerline on the base leg of the pattern.

They basically ask you to report it as a "memory jogger" for themselves so that they can do something when you call in (like clear you to land).
A standard airport traffic pattern is described in Chapter 7 of the Airplane Flying Handbook as:

The downwind leg is a course flown parallel to the landing runway, but
  in a direction opposite to the intended landing direction. This leg
  should be approximately 1/2 to 1 mile out from the landing runway, and
  at the specified traffic pattern altitude. During this leg, the before
  landing check should be completed and the landing gear extended if
  retractable. Pattern altitude should be maintained until abeam the
  approach end of the landing runway. At this point, power should be
  reduced and a descent begun. The downwind leg continues past a point
  abeam the approach end of the runway to a point approximately 45° from
  the approach end of the runway, and a medium bank turn is made onto
  the base leg.

It looks like this:

When the tower asks you to fly a two mile pattern, they want you to fly a pattern that has the downwind leg 2 miles out from the runway instead of the 1/2 to 1 mile that is normal.  The base leg would still be turned when 45° from the approach end of the runway.
In this case, you are either:

Doing pattern work in which case you fly the two mile wide pattern and should report turning base, or
Approaching the airport and the controller would like you to enter the two mile wide pattern on the base leg.  You should report the point where you would have normally turned onto the base leg if you were flying the entire pattern (45° to the runway).  This would actually be both of your last two scenarios above:  Established on the base leg, two miles prior to intercepting a two mile final (which for fun would actually be 2.83 miles from the runway).

I've actually always been told the opposite - a "2 mile base" means you are on the base leg that would put you onto a 2 mile final after your base-to-final turn.

@egid That isn't the opposite of what I said, it is exactly what I said (only much shorter, lol).

Ah, yeah, rereading I see that. You seem to imply that it also requires a wider pattern by mentioning the 45° turn.

@egid 45° is just how they define where the turn is supposed to be.

I can see how @egid was confused by what you'd said, "When the tower asks you to fly a two mile pattern, they want you to fly a pattern that has the downwind leg 2 miles from the runway" seems like your downwind is 2 miles from the extended runway centreline.

@Qantas94Heavy Umm, that is what I'm saying....  2 miles from the runway (and parallel to it, which is what the downwind leg would be) would also be 2 miles from the extended runway centerline.

That's to much math, guys ;) - If the controller tells you to report a two mile base, plan your base two have a twice as long final as usual and give him a call, when you are on that base. They probably don't expect you to read an exact distance from your GPS. It's just to separate you from traffic flying a normal pattern or create some spacing on final and for them to have a heads up when yo are close.

@Falk Math?  What math?  :)

@Lnafziger what math? well, you know: 2x1=2

I'm skeptical of this answer, and always thought a "2 mile left/right base" meant what you'd get if you took a normal pattern and extended the base leg out for 2 miles. I.e., it would put you on a normal final. The quote given does not confirm (or refute) this answer; is there an authoritative source to confirm this answer?

The confusion here (and elsewhere on the internet) stems from this being non-standard phraseology. Either interpretation is completely justifiable, therefore neither interpretation is "correct." As ATC, a better option is to be explicit: "Enter/Report left base for a two-mile final" in one case or "left base two miles from standard final" in the other.

If you're asking about the usa, the FAA somewhat recently updated the Pilot/Controller Glossary term "TRAFFIC PATTERN." The term now includes the following notes:

NOTE-
ATC may instruct a pilot to report a “2−mile left base” to Runway 22. This instruction means that the pilot is expected to maneuver their aircraft into a left base leg that will intercept a straight−in final 2 miles from the approach end of Runway 22 and advise ATC.

NOTE-
ATC may instruct a pilot to report “5−mile final” to Runway 22. This instruction means that the pilot should maneuver their aircraft onto a straight−in final and advise ATC when they are five miles from the approach end of Runway 22.

So, there's no implication about where the downwind leg should be -- no implication that the downwind-to-base turn should be the normal 45-degree angle from the approach end of the runway-- and in fact no implication that there even needs to be a downwind leg at all?  And is there in fact an implication that there should not be a downwind leg?

The implication is that there should not be a downwind leg. As used in the note, "report two-mile base" is shorthand for "enter and report reaching a two-mile base."


text:
Why are we still putting lead in our fuel?
Lead in gasoline for automobiles has been banned from nearly every corner of the world, with most bans dating from the nineties. Why are we still poisoning ourselves with lead in Avgas? I understand that Tetraethyllead (as it's apparently technically called) is an octane booster, but the Wikipedia article also mentions that advances in the production of high octane unleaded fuel have pretty much nullified that argument. There are more reasons of course, but none the automotive industry wasn't unable to overcome many decades ago. I know it's 'low-lead' with very little lead compared to 'traditional' gasoline, but the phase-out of leaded fuels in cars started back in the 1970s, so don't tell me that most aircraft are simply too old. Are aircraft engines so special compared to car engines, that they must get their fix of lead before they go flying?
It is also a fact that there are aircraft piston engines which do run perfectly fine on unleaded fuel, why is it that hardly any airport offers it? Even in cost-conscious and eco-friendly Europe, most airports offer only 100LL and Jet fuel.

wiki (last sentence of the paragraph) says it's because of pricing

@ratchetfreak; it does indeed say that, however, unleaded high octane fuel is still cheaper. I think they're looking for an alternative to Tetraethyllead which is economically feasible, that is, for engines which require leaded fuel, it doesn't talk about why said engines require (and continue to require) leaded fuel.

there is little stopping you from installing a car engine in you cesna and using mogas (besides need for space, water-cooling, lack of safety certificate,...)

@ratchetfreak The Katana I fly is certified for, and runs perfectly well on Mogas (it's a Rotax). The question is still, why are lead-fuel-engines still being produced today, more than 40 years after the phase out of lead-fuel-engines began, and more than a decade after the use has been banned in all other circumstances. It can't be that they're old. Take the Continental IO-550 for example, it was first run in '83, it has a FADEC for crying out loud, and still munches lead.

just as a comment because of lack of time: engine certification is just so expensive that no producer can be bothered while their standard 30-year old products are still selling. Rotax is different because they open up a new market (Ultralight and VLA).

@yankeekilo; Good point, but wouldn't the IO-550 need certification? Or is there a fast-track if it's "similar enough" to old engines or something?

@roe not sure about that, but the entry threshold for new players is extremely high, which is why development is incremental at best (as so often in aerospace).

Shell has announced a replacement.  10 years in the making from a company with very deep pockets and a lot of PhDs.  Probably not easy to do.

If anyone's interested, http://www.avweb.com/avwebflash/news/New-Petition-Calls-For-Action-On-Avgas-Lead221923-1.html

There is nothing "special" about an aircraft engine that requires lead in the fuel -- Engines don't much care about the lead in tetraethyl lead, nor do they much care for it (it gets everywhere, fouling sparkplugs and contaminating the oil). What they care about is the octane (anti-knock) rating of the fuel.
Many "low performance" aircraft engines run just fine on unleaded fuels with a lower octane rating (among them the Lycoming O-320 and O-360 family that power a good chunk of the GA fleet), but high-performance aircraft engines (e.g. the IO-520 you'll find in Beech Bonanzas) require a 100 octane fuel. The approved specification for 100 octane aviation gasoline (ASTM D910) lists tetraethyl lead as the octane booster of choice.
It's also worth noting that some of the octane boosting techniques used in automotive gasoline are not acceptable for aircraft engines (the most common in the US being the addition of ethanol, which has two undesirable effects: reducing the energy content of the fuel, and damaging aviation fuel system seals and other components).

So why do we still make 100LL, and why don't we offer the other unleaded options at every airport?
Aviation gasoline is a minuscule slice of the gasoline market, so it doesn't make sense to have 5 tanks with 5 different grades of fuel at every airport:
All aircraft engines will run on 100LL, so 100LL (leaded) aviation gasoline is still produced because it is a "single fuel" solution to piston aviation's needs.
If leaded avgas were to disappear tomorrow the engines that require it would be left without an approved fuel, which would result in those aircraft being grounded until such time as an alternative fuel could be developed or the engine manufacturers develop a procedure for derating the engines (operating them at reduced power). For obvious reasons neither of these options is attractive, particularly to folks who own higher-power engines which were presumably purchased for the performance...
Changing the avgas specification is quite a bit of work - it requires ensuring that the new fuel is a "drop-in replacement" for 100LL -- one which can be mixed with 100LL in any proportion, and will work correctly in any engine designed for 100LL fuel.
The FAA is working with engine manufacturers and major fuel producers on a program which will result in such a specification (a Google search for unleaded avgas transition also produces useful results), and hopefully a universal unleaded aviation fuel specification and one or more products will come out of those efforts.

Although a question remains: why do most aero piston engines seem so outdated compared to modern automotive counterparts?

Good answer, but lead also acts as a lubricant and some engines depend on that, so your first paragraph isn't really 100% accurate.

@yankeekilo That has a lot to do with the process of getting anything through FAA certification -- modernizing an engine would require a mountain of paperwork, and specific approvals (probably in the form of STCs) to install it on certificated airframes as a replacement for the "old" designs.  The existing "vintage" engines work, so we keep producing them (just like the existing aircraft designs work -- today's newest Piper Archers are largely identical to the first Cherokees produced...)

@Lnafziger Personally I don't put much stock in "lead as a lubricant" - I think the gunk from lead build-up is just as likely to cause sticking valves as it is to help  lubricate them. Where lead normally comes into play is the deposits on valves/valve seats acting to soften the impact of the valve on the seat, preventing valve seat damage ("valve seat recession"),  which is actually something to consider, but I doubt many aircraft are flying around with original "soft" valve seats at this point. Might be an issue for "classic" big-bore radial engines though.

Well, too much lead will absolutely case sticking valves, but in aircraft that were designed for leaded fuel, completely eliminating it can cause problems as well.

There is also tendency to use diesel engines burning Jet-A/Jet-A1 for the lower power engines and turboprops (burning also Jet-A/Jet-A1) for the higher powered ones and get rid of avgas altogether (because it's already more expensive and less available, at least outside US).

"[ethanol is] damaging aviation fuel system seals and other components", that has got to be just a matter of choice of materials, or otherwise cars all around would be falling apart, wouldn't they? Or are there specific components not used in cars, like a fuel bladder or something?

@roe My understanding is some of the problem components are the fuel bladders & fuel pump diaphragms - It's definitely a matter of materials choice there (though changing the materials would likely require recertification of components), but ethanol in fuel has other unattractive qualities that make me decidedly "not a fan" :)

@roe - lol, ever been to a car repair shop? Cars all over the world are definitely falling apart... Okay, okay, I'm being silly :).  But, keep in mind, aircraft engines do have to be far more reliable than an automobile engine.  If an automobile engine quits out on you, you roll to a stop and call a tow truck.  In a plane if your the engine cuts out, or even loses power, you may well die.  So the engine has to be much more precise.  Hence the FAA is picky about engines, hence it's taking a lot of time to get regular 100 out the door.

@falstro, ethanol will destroy rubber seals, gaskets and hoses very quickly. Cars approved for E-85 have stainless steel hoses and different seals to resist the ethanol. On the flip side, huge power can be made from ethanol on the road, or methanol/ethanol on the track when the entire fuel system is designed for alcohol.

@FreeMan sure, ok. So why are we using fuel systems that can't handle ethanol? Most of the airplanes in use today were built long after lead was banned on the road.

@falstro three little letters: FAA (as voretaq7 addressed).

@FreeMan Ok, so you agree that the point has been discussed already? Blaming the FAA is a bit of a cop out as there are several certified engines as well as a bunch of STCs for existing engines/fuel systems. Not to mention that it doesn't cost the FAA (or EPA or whatever) a dime to say "ok guys, listen up, tomorrow/next year/decade leaded fuel will no longer be legal".

@falstro Yeah, sorry, commented before I realized how old the question was.

Lead in fuel is primarily an anti-knock agent, raising the octane rating and permitting higher compression ratios in the engine which in turn provides more power and more efficient use of fuel.  Many GA aircraft engines are built specifically to take advantage of the anti-knock, higher octane fuel.  Although there is a lot of work underway to develop a non-leaded substitute fuel that will allow the same performance, we are not there yet.  There is no alternative fuel certified for these engines, and burning unleaded fuel at high power settings will damage these engines severely and quickly.  So we still need the lead in these cases.
With luck and technology, this will change in a few years.  If not, diesel engines are a likely replacement for the 100LL burners.  But that is a pricey change for most GA planes.

My understanding is that it is simply a case of certification.  There is no replacement certified fuel that will take the place of 100LL.

One factor may well be that engines that run on leaded fuel are more fuel efficient than those that don't.
IOW you get more horsepower and/or better range from leaded fuel than from an identical amount of unleaded fuel when burnt in an engine with similar level of technology.
So an aircraft burning unleaded fuel would have worse performance and/or worse range. And of course especially the performance would cause trouble as it means longer takeoff run, lower service ceiling, worse climb performance, and potentially leaving you with an aircraft that can't even get off the ground.
And given the far longer lifespan of aviation engines as compared to car engines, there are far more aircraft around that need leaded fuel, the transition period would be far longer as well. And a car where you put a bottle of lead replacement compound in the fueltank, if it drives a bit slower it's not as bad as an aircraft where you're doing the same and it's not just slower, but can't climb to the same altitude and needs more runway...

There have been examples of very efficient, Mogas-compatible flight engines, like the Porsche PFM 3200, so the efficiency angle may not be always relevant. Modern car engines are much more efficient than their lead-guzzling counterparts from just 15 years ago...

@yankeekilo yes, but a car engine with similar fuel consumption to one using leaded gasoline created at the same time would not be. That's the whole point. There are so many aircraft out there using engines that require leaded fuel, and not just museum pieces but in operational use, that would require new engines to be designed for them, it's just not feasible.

Current aviation fuel has 1 part per million lead in it; and is absolutely critical for cooling aircraft engine exhaust valves.  Or, if you burn unleaded avgas without an additive; you will eventually burn one or more exhaust valves.  In a 4 cylinder engine loosing a cylinder due to a burned exhaust valve could put the aircraft and the pilot in a deadly situation.  6 cylinders or more; loosing one cylinder due to exhaust valve burn will still get you back to the airport.  Air cooled aircraft engines absolutely need that one part per million lead to keep the exhaust valves from overheating and loosing that cylinder.  Anyone who has flown on unleaded fuel for any length of time has probably burned and exhaust valve.  I know, because it happened to me twice. And since there are are no more that a few hundred thousand non turbine aircraft on Earth, keeping the lead in the avgas will save lives and with such a small number of aircraft compared to autos; leaving lead alone in avgas should be no brainer. Diesel fuel specifications generally allow 20 parts per million lead in the diesel fuel; naturally from the Earth where the crude diesel was taken.  And there are several hundred million diesel engines on Earth creating a lead in the air problem at least 1000 times greater than aircraft one part per million leaded avgas.  Tell everyone who asks; leave that one part per million lead in avgas to save lives by not burning aircraft engine exhaust valves.

Although a question remains: why do most aero piston engines seem so outdated compared to modern automotive counterparts? – yankeekilo Mar 3 '14 at 18:26

This answer is factually incorrect. 100LL is specified at ~500 ppm, not 1 ppm. Example: BP 100LL spec sheet, you can look up others. The lead does nothing for cooling the exhaust valves. It's the octane rating that helps, not the lead.

Also diesel doesn't have lead


text:
Why is the altimeter unwinding unevenely during a spin?
Spencer Suderman recently did a world record 81-turn inverted flat spin, and dropped over 21 thousand feet while doing so. The entire thing is documented on youtube. The spin starts at 3 minutes into the clip. Looking at the footage, in the beginning of the spin, and also (although to a lesser extent) at the end of the spin, the altimeter is unwinding very unevenly, being almost steady for a second and then quickly unwinding 3-500 feet before steadying (or actually increasing the first couple of turns) for another second. Why is that? It seems unreasonable to me that it would actually be leveling out.
It's worth noting that in the middle of descent, when spin is perfectly flat, it's unwinding evenly.

turbulence over the static ports I think

The altimeter works by measurements of the pressure in undisturbed air, also called static pressure. A static port is placed, usually on the side of the hull, perpendicular to the airflow.
During a spin, the airflow will not flow evenly past the static port, but will occasionally flow directly onto the static port. This will cause a rise in measured pressure, leading the altimeter to believe it is lower than it actually is. This cause fluctuations in the indicated altitude.

Some aircraft have multiple static ports to help average out this kind of situation (it's generally intended to deal with the uneven pressure when flying in a slip, but conceivably it would help in this situation as well).

DeltaLima's answer is correct, but he doesn't say why there are oscillations. If you watch the horizon, you will see that initially and at the end of the spin the roll angle oscillates, whereas in the middle of the spin the roll angle is steady. The uneven altimeter correlates nicely with this roll oscillation. I assume that the static ports are at the rear fuselage, and during a spin the airflow will have a noticeable sideward component there. This will influence the pressure on the static port, and an oscillating change in this sideward component causes the halting movement of the altimeter.


text:
Is the phrase "going missed" standard in the US?
The use of the phrase "going around" is specified by ICAO Doc 4444 as the phrase to use when we're aborting the landing and heading for

another lap in the traffic pattern if we're on a visual or VFR approach, or
the appropriate missed approach procedure if we're on an instrument approach

However, in the US, I often hear the pilot saying "going missed" when breaking off an instrument approach. Is this standard phraseology in the US, or another one of those non-standard phrases which have gained footing? The 7110.65 only mentions the instruction "go around" (which incidentally is analogous, traffic pattern if visual, missed approach if on instruments), but doesn't say anything about the appropriate pilot phraseology. I couldn't find any reference in the AIM (chapter 4 section 2) either.
(I also often hear just "missed approach", which I suppose would be appropriate when checking back in with approach, but not with the tower, although feel free to clarify that for me as well)

FWIW, I always used "missed approach" to say that I was aborting the approach regardless of whom I was addressing. In other words the approach was what I had missed rather than who I was addressing. Also, it was always used in relation to aborting an instrument approach. "Going around" was the phrase for visual usage. Interestingly, at least to me, I never heard "going missed", which makes me wonder if that phraseology has come into vogue only since I retired 15 years ago.

"Going missed" is slang. The points of standard phraseology have been discussed at length in many places but a key factor is often missed. ATCOs are trained and conditioned to work with the standard phrases. In high stress situations, or perhaps with a lot of traffic, standard phrases which indicate something outside the norm tend to penetrate the noise and register. 99% of the time, a go around is a non-event but there could be situations where a go around could affect safety. Going around" is no harder to say than "going missed" so why short circuit one extra little safety factor?

The US has no document/order/regulation for proper phraseology by pilots.  ATC must follow 7110.65 but there is no analog for pilots.

@Terry: In an instrument approach when aborting when already visual and below minima, would you then call it "going around" or still "missed approach"? (not that there would be much practical difference of course)

@JanHudec I honestly don't have an answer for that. Never had that situation. When giving instrument instruction, I always had the student do a purposeful "missed approach" at the minimums and make that call to the frequency we had been handed off to, which sometimes was the tower and sometimes approach or departure control. In real (as opposed to instructional) conditions, I never had occasion to not land once below minimums. My guess is that, coming off an instrument approach, I would have used "missed approach."

@JanHudec In thinking about it further, saying "missed approach" would have meant I would fly the published missed approach procedure or comply with what ATC had previously said to do in case of a missed approach. So, if aborting meant not being able to proceed with VFR minimums, "missed approach" would be good. If there were VFR mins, I'd like to think I would have said "going around." Or, if below VFR but I could maintain visual contact with the runway, perhaps something like "going around, request contact approach to runway xx."

According to the Pilot/Controller Glossary (the closest thing to "regulatory" documentation regarding phrases pilots are supposed to use on the radio) there is no GOING MISSED - there is GO AROUND as an ATC instruction (and by analog GOING AROUND as a pilot's response or notification to ATC to inform them the pilot is doing so), which covers both VFR and IFR operations:

GO AROUND −
  Instructions for a pilot to abandon his/her approach to landing. Additional instructions may follow. Unless otherwise advised by ATC, a VFR aircraft or an aircraft conducting visual approach should overfly the runway while climbing to traffic pattern altitude and enter the traffic pattern via the crosswind leg. A pilot on an IFR flight plan making an instrument approach should execute the published missed approach procedure or proceed as instructed by ATC; e.g., “Go around” (additional instructions if required).

(There is also an IFR-specific EXECUTE MISSED APPROACH in there.)

GOING MISSED is not proper/correct phraseology (though much like "tally one" or "got 'em" in place of "traffic in sight" it will generally be understood by ATC - probably with about the same grinding of teeth as the other two phrases get).
Informing ATC (tower or approach) that you are executing a missed approach by simply saying N12345, Missed Approach is correct phraseology however:

MISSED APPROACH−
  a. …
  b. A term used by the pilot to inform ATC that he/she is executing the missed approach.
  c. …

and is probably what should be used in place of "Going Missed".

For what it's worth I don't think I've ever heard ATC instruct an IFR flight to just "Go Around" - though I have heard "Go around, make (left/right) traffic", and "Go around, execute missed approach". Presumably they do that to avoid the ambiguity with IFR traffic ("Should I go around and join the active traffic pattern, or should I execute the missed approach procedure and come back?")

If I read it correctly, "Execute missed approach" can only be used as either a) an instruction during a PAR approach, or b) as part of an advisory statement, leaving the decision to the PIC.

@roe It's possible I'm misinterpreting it, but my understanding is "Execute missed approach" is applicable to all instrument approaches. The distinction for PAR/ASR approaches you execute the missed approach procedure  (climb/turn) immediately when told to do so (the assumption being that ATC can no longer provide safe radar guidance), whereas for other types of approaches you would continue inbound to the missed approach point using your onboard navigation equipment, then execute the missed approach procedure from that point.


text:
Do instrument approach procedures sometimes have a minimum temperature?
Non-precision instrument approaches generally have altitude restrictions which get lower when you get closer to the airport. I always figured these restrictions were AMSL using the current altimeter setting, not compensating for temperature.
Some have heard the mnemonic that mountains are higher come wintertime, which basically means that colder weather make your altimeter read higher than you actually are (or, as most pilots prefer to think, you're lower than what your altimeter reads)
Have a look at this VOR approach into Newark

Most altitude restrictions are a minimum level, so you're free to fly higher if it's a particularly cold day. But have a look at LOCKI intersection, the final approach fix. That's at 1500 ft, not at-or-above. At -40, this will put you around 1100 ft above ground level. Although I don't see any obstructions that high during this segment of the approach, as far as I know instrument approaches are supposed to guarantee a 500 ft obstacle clearance, do procedures using this kind of restrictions have a minimum temperature?

There are procedures with temperature restrictions, related to altitude constraints. An example is Innsbruck:

The text in the red box says:

Procedure N/A below AD temp -7°

For effect of temperature on altimeter: How will the altimeter read in air colder than ISA?

Do you know whether they have separate procedure for use if AD temp is below -7°C? Because it's not like it would be rare temperature there during winter.

@JanHudec There is another procedure available, however than one requires better visibility.

It is true, that from HIGH TO LOW, LOOK OUT BELOW is both for pressure and temperature and the colder it is the lower one is going to fly.
This error in the altimeter is corrected two ways.

The local altimeter setting will correct for both non-standard pressure and non-standard temperature at the field itself.  The altimeter should read field elevation (within 75 feet) when on the ground.

The altimeter is designed to use a standard temperature model.  As the actual temperature deviates from that model errors will occur.  This is corrected for by adjusting the altitude for non-standard temperatures.

Canadian pilots are tested on applying these corrections anytime the temperature is below zero.
U.S. pilots are required to reference a document called "Cold Weather Restricted Airports" that lists which airports have obstacle clearance issues using coldest known temperature data over the previous five years for each airport.  The link is found on the FAA d-TPP website.
KEWR is not currently listed as a cold weather restricted airport.  If it was, there would be a note on the plate showing an inverse snowflake and a temperature at which it must be compensated.
U.S. AIM Chapter 7 Section 3 has a great discussion on the issue and a correction table for non-standard temperatures.
https://www.faa.gov/air_traffic/publications/atpubs/aim_html/chap7_section_3.html

The design of the procedure considers the weather data for the location. Newark does not have -40 temperatures.
This only affects procedures using barometric altitude. ILS and augmented GPSS are not affected and would be alternates.

Average coldest temperature (ACT). A value in Centigrade (°C) and/or Fahrenheit (°F) scale for the lowest temperature a Baro‐VNAV (including RNP) procedure can be utilized. It is derived from historical weather data, or in the absence of historical data, a standardized temperature value below airport ISA is used.

There can't be a fixed 500ft clearance along the whole approach because some approaches have minimums below 500ft or follow descending terrain to the airport. Part of that 500ft, where it is used, is to account for temperature. The pilot test standard for altitude on the final approach segment is -0+100 for a reason.
As for the details, procedure design is complex and it would take a full college semester just to cover the basics. FAA documents in the 8260 series cover a lot of it(hundreds of pages each), but they are written for internal use not as guides for non-specialists. They also refer to the results of custom engineering software that they use, and you would need access to the source code to know its methods and parameters.


text:
How hot would pressurized air get if the air conditioning failed?
I understand that aircraft are pressurized by pushing air into the cabin using the engines, and using a valve to let the air escape at a controllable rate thus achieving the desired pressure differential. However, when air is compressed, it heats up and if I understand correctly, all pressurized aircraft have air conditioning units to keep the temperatures at a reasonable level.
If all air conditioning units were to fail, how hot would the air being shoved into the cabin actually be? If this depends on aircraft and or engine type, any example as a ballpark figure would be appreciated.

In reality, heat loss through the air ducting and cabin insulation is so strong that cabin temperatures tend to be lower than what is considered comfortable. In order to save fuel, on many airlines the air flow into the cabin is reduced, and the little heat this air carries into the cabin is not enough to keep the cabin warm enough.

@PeterKämpf do you have any references to back that claim? It would appear from the answers that it isn't accurate, and pressurizing the air would bring it up to several hundred degrees which is a lot to loose by just piping it into the cabin.

@PeterKämpf then why am I always uncomfortably hot when traveling on an airliner?

@jwentig: I don't know. I am always too cold, especially on long haul flights. The heat would be there if the cabin is pressurized instantly, but this is not what happens in reality. Not considering the heat losses gives totally misleading answers (as those to this question nicely demonstrate).

@PeterKämpf what do you mean "[not] pressurized instantly"? Air is taken from outside, and compressed to the pressure inside the cabin; that's a pretty quick process, wouldn't you say?

@PeterKämpf I have been in two different airplanes where the packs have failed, and I can assure you that the air coming into the cabin gets quite hot!

@falstro: I do not dispute that the air out of the ACM is hot. But this heat can only be felt when the air flow from the ACM fills the whole cabin instantly. In reality, the flow is just enough to replenish what is leaked away plus some amount for ventilation. Thus, the heat is dissipated in the cabin air, and the big surface area of the cabin cools the whole cabin air down. If the ventilating flow is low, the cabin air becomes really cold, given an outside temperature of -50°C and lower.

The air conditioning is in a so called "air cycle machine" (ACM). If it fails, the air will not be compressed (done in the same machine).
Temperature goes up and down as the air passes through the machine's heat exchangers, compressors and turbines, reaching 250$^\circ$C at peak.
Wikipedia article

Image from ECS

For most airplanes the air entering the ACM comes is already hot and compressed from its source.  E.g the EMB-145 draws bleed air from the 9th or 14th compressor stages of the engines (or the APU).  This is similar in other airplanes, though the particular compressor stages may differ.  If the ACM was inop, the resulting cabin temperature should have the same equivalent potential temperature ($\theta_e$) as the ambient atmospheric air.

The exact values depend on the specific aircraft and engine setup, of course, but I'll talk about a 767-300ER with P&W4000 engines at cruise altitude.
The first thing to take in to account is Gay-Lussac's law
\begin{equation}
\frac{P_1}{T_1}=\frac{P_2}{T_2}
\end{equation}
The pressure outside the aircraft @ cruise is around -70°F (~216 K) and 2.9 psi (~20 kPa). The pressure inside the aircraft (we'll talk about the compressing stage in a moment) is around 11.5 psi (~79.3 kPa).
\begin{equation}
\frac{20\ kPa}{216\ K}=\frac{79.3\ kPa}{T_2}
\end{equation}
\begin{equation}
T_2 = 856.44\ K (1082°F)
\end{equation}
In reality, its a bit more complicated than this, because this assumes that the volume of air is constant, which in this case it is not. In the low-stage compressor, the air is compressed to 30 psi (~206.8 kPa) and 400°F (~477.6 K). That is what is piped through the bleed valve and into the air conditioning units and is most likely what the temperature of the air would be if the bleed valve was closed and the air conditioning units were disabled.
Source (PDF Warning)

$1082°F\approx583°C$

This is in the neighborhood of the bleed air from the engine, but it goes through a precooler before being used for things like wing anti-ice or the AC packs.

what nobody is considering here at all is the bleed air is highly compressed when taken from the engine. 400 to 600 degrees Celsius are not uncommon.
But, what happens when you expand gases? Yes the opposite than when compressing it, it cools down. So the it would even without cooling get close to a reasonable temperature and, since the plane is not pressurized same level as the bleed air is in the engine before leaving it.
And as somebody mentioned in a comment, "the little air exchanged" in the cabin. In most airliners today the air inside the cabin is completely exchanged every around 3 to 4 minutes, so the airflow is quite considerable.

Another thing that no one seems to have considered is that the heat in the cabin of an airliner is mostly generated by its human cargo.  It's that heat that the A/C system needs to remove, just as it would need to remove the heat from that many humans in a room on the ground.


text:
How do you check the correct operation of the rudder when it's tied to the nose wheel?
Preflight checklists often contain the item

Flight controls: Free and correct

However, if the rudder is tied to nose-wheel steering, is it even possible to check if the rudder is correctly connected (unless have a rear window and can see the rudder during taxi)?

How would you determine that the rudder is correctly connected even if it isn't tied to nose-wheel steering?  The nose-wheel/rudder issue and the free & correct issue seem like two different questions to me.

@Lnafziger you push the pedal and stick your head out the side to see if it's turning the right direction?

Every small airplane that I've flown that uses the rudder pedals for nose wheel steering uses bungee cords or springs so that they will stretch when pushing the pedals without the aircraft moving.  This allows the pedals to move without requiring the nose wheel to do so as well, and you can check them in the normal manner.
Larger airplanes that use the rudder pedals for nose wheel steering have a method of turning the nose wheel steering on and off, so the check could be done before the steering is engaged.  That being said, most larger aircraft do the control check during the taxi (that's when most airliners are doing it) and there is nobody out there to make sure that the rudder is actually moving the correct direction.  In my airplane, the response for the flight controls is "free and self-centering" (since we have hydraulic flight controls) and we don't verify the actual direction of rudder movement.  That's a maintenance function.

Cool. I just had a deja vu.. Have I already asked this question? I need to check if the two types I fly with nose wheel steering (Dynamic WT9 and Socata TB10) have bungee cords. The rudder pedals feel very stiff when sitting still, how hard would you have to push?

Usually not too hard, and you wouldn't want to force it because if they aren't bungees you could break something.  I've never flown either of those though so I have no idea!

@Lnafziger The PA-28 series does not use bungees (the nose wheel is connected to the rudder pedal by rods). As with most airliners verifying correct rigging of the rudder on those planes is a maintenance item (done with the nose wheel jacked up off the ground).

Every plane I've ever flown with a linked nose gear, the rudder will make noise when it hits the stops. On top of that, the nose gear linkage is disconnected when you use more than a certain degree of deflection. If you push the pedals past that point and feel resistance, your rudder is working.

This might tell you that it's hooked up to the cables, but not which way around? Or is there some kind of trick?

@falstro if they are hooked up the wrong way then maintenance should have tested that before releasing the plane for operation

@ratchetfreak while true, there have been accidents where ailerons were reversed; which is why you check that they are moving the right way during preflight :)

@falstro do a check on your roll-out. Pull the nose wheel off the ground like a soft-field takeoff and wiggle the rudder a bit. Other than that, you're just going to have to have someone stand outside and look I guess.

It depends on the construction of the aircraft, if you have a linked nosewheel you generally cannot check this in preflight. Once you start up and are taxiing you would have a look out the back and check. When turning left I check compass, di, turn/bank all showing the correct response and rudder left. Vice versa for turning right.


text:
Do autopilots in non-glass cockpits feed off of the gyro instruments?
Do autopilots in non-glass cockpits feed off of the gyro instruments to determine aircraft attitude, heading and rate of turn? Or do they have their own internal gyros? Since I can set the heading using the heading bug, I suppose it needs to be able to read the heading-bug offset at the very least, so there's some connection.

Why do you think there would be difference between glass and non-glass cockpit?

@JanHudec I clarified in a comment to Emil's answer "I deliberately made the distinction non-glass cockpit, as [in a glass cockpit] the sensors are designed to be read electronically. In a non-glass cockpit the sensors are primarily read mechanically, with the gyro physically touching some mechanism moving the 'little airplane' around. Do you have any idea how this information gets conveyed to the autopilot? Is it standardized?"

Yes, they do. Also for glass cockpit, the autopilot uses the sensors that are used for the displays (ADC, AHRS, etc).
Here are some extracts from the FLM of the SA 365N Dolphin, describing the autopilot and coupler (I selected only parts relevant to the question):

Hello Emil, welcome to Aviation.SE. Can you provide any reference to your answer? While I believe your answer basically is right, the answer would benefit from some background / links.

@DeltaLima: I added some extracts from the flight manual of a analog instruments helicopter with AP. Unfortunately the description in the manual is not very detailed (there are no schematics).

Hi Emil! Thanks, I deliberately made the distinction non-glass cockpit, as there the sensors are designed to be read electronically. In a non-glass cockpit the sensors are primarily read mechanically, with the gyro physically touching some mechanism moving the 'little airplane' around. Do you have any idea how this information gets conveyed to the autopilot? Is it standardized?

@falstro Typically there's some kind of electrical pick-up inside the gyro, connected to the same mechanism driving the display (this rather old one is a rate-of-turn indicator, and you can see the wound resistor and wiper arm on the left side of the photo -- Based on the resistance the autopilot knows what the rate-of-turn indicator is showing, and can act accordingly). Several designs exist, designed to work with different autopilots.

Typically the Turn Coordinator and DG is used. Sometimes the Attitude indicator is used as well. Of course, if NAV tracking is used the OBS or GPS has in input as well.